Interconnection of discrete bare chip components or microelectronic components (chips) on a substrate is a process for which precision material deposition with sufficient accuracy is necessary to allow for miniature electrical or other type of interconnects so that when a chip component becomes attached to a placement position all electrical connection and fixation to the substrate can be made. Typically, it is foreseen to carry out this process by bonding bare dies via an isotropic conductive adhesive or solder pastes face down, i.e. with the electrodes at the bottom of the chip or package pointing to the circuit board electrodes. An example of a conductive adhesives is a thermal or UV settable resin loaded with silver particles. A solder paste typically comprises solder particles and flux. Such solder pastes require flux for removing the oxide layers on the particles and improve wettability during reflow. Both types of interconnections, i.e. conductive adhesives or solder pastes, are sensitive to thermal shock that should be avoided before chip placement. Flux is activated at temperatures above 120 and 150° C. and also for a conductive adhesive a thermal shock will degrade the adhesion properties of the adhesive. Currently there is known a number of methods for printing conductive adhesives and solder pastes for bonding bare silicon or LED chips, interposers or ball grid arrays on flexible substrates. The state-of-the-art methods includes screen printing and stencil printing. The current techniques are efficient; however, they suffer from certain intrinsic limitations.
a) Screen and stencil printing are fast techniques, however they don't have the necessary resolution (100 um approx.). The contact mode and the web movements of the substrate make it prone to dislocation. As it is a contact mode process, damage to fragile substrates is possible and only a single layer of material can be deposited before chip placement. It cannot handle non-flat surfaces and especially for foil based roll to roll processes web deformation cannot be compensated. In addition, fabrication of a screen is expensive, not very flexible and they have to be replaced after 10,000 to 100,000 runs for screen printing and 200,000 for stencil printing. Finally regular cleaning and maintenance is required for proper results.
b) Dispensing and jetting are non-contact methods and do not require stencils or masks. Their resolution is however limited to 250 um which is not sufficient for most bare die Si chips and miniature passive components. In addition they are relatively slow processes with typically throughputs of 10 dots per second.
c) Pintransfer is a possible method however it is restricted in viscosity range and thicknesses of the layers to be transferred and not flexible in the shapes to be transferred
In “Three-Dimensional Printing of Interconnects by Laser Direct-Write of Silver Nanopastes” Piqué et al, Advanced Materials Volume 22, Issue 40, pages 4462-4466, Oct. 25, 2010 a laser direct write method is illustrated of a solvent based, dried nanosilver paste that is transferred to a substrate, in order to provide an alternative method for electrical wire bonding interconnects for bare die LEDs. However, this method is not a face down bonding method wherein electrical connections on the chip and on the substrate are pointing in the same direction and it also does not provide any fixation by adhesion to the substrate. It is inadequate since it provides only an electrical connections after placement of the bare die in pockets on a polyimide substrate but no structural adhesion. Moreover, this method is not suitable for most interconnect materials such as thermosettable conductive adhesives as using a laser pulse will cure or degrade the adhesive rendering it unsuited for bonding.